Ringworm or Dermatophytosis in Dogs and Cats
Dermatophytosis is a condition of dogs and cats (or people, or other animals) having skin, nails or hair infected by certain fungi.
The condition is commonly called by the ancient term “ringworm,” because, centuries ago, people thought there was a worm living in the circular lesions the fungus sometimes produces. There is no worm associated with ringworm infection.
Dermatophytosis is an infectious and contagious superficial fungal skin disease of dogs and cats. The disease is not life threatening and will resolve without intervention; however, treatment is recommended to shorten the course of infection and limit transmission. DERMATOPHYTOSIS is a superficial fungal disease of the skin and hair of cats and dogs. In small animals it is most commonly caused by Microsporum canis, Trichophyton species and Microsporum gypseum. Trichophyton and Microsporum are being reclassified into the genus Arthroderma .
Dermatophytosis is a highly contagious fungal infection that can occur in humans, dogs and cats. It is commonly called ringworm but there are no parasitic worms involved. The fungal spores shed from an infected animal can live in the environment for over 18 months. Dermatophytosis fungus is more common in hot and humid climates.
The fungi that causes dermatophytosis belong to a specialized group known as dermatophytes. Dermatophytosis fungi feed on the keratin found in the outer layer of the skin, hair and nails. This fungal disease usually infects the hair follicles of the canine. It causes the hair to break off at the skin line, resulting in round patches of hair loss. As the disease spreads over the dog’s body, the lesions may be irregular shaped. Dermatophytosis is not usually itchy but the lesions may lead to skin sores caused by your dog licking at the infected area.
Many people are surprised to find that ringworm is not caused by a worm at all, but by a fungus. The fungi involved are called dermatophytes, and the more scientifically correct name for ringworm is dermatophytosis. The dermatophyte fungi feed upon the dead cells of skin and hair causing in people a classic round, red lesion with a ring of scale around the edges and normal recovering skin in the center. Because the ring of irritated, itchy skin looked like a worm, the infection was erroneously named. The characteristic ring appearance is primarily a human phenomenon. In animals, ringworm frequently looks like a dry, grey, scaly patch but can also mimic any other skin lesion and have any appearance.
Some dogs may be asymptomatic carriers, which means they do not show outward symptoms of the disease but are able to transmit the disease to other animals or humans.
If you believe that your pet has dermatophytosis, it is imperative that you take him to a veterinarian to help ensure the disease does is not transmitted to other members of your household.
Dermatophytosis is a fungal infection that affects the superficial layers of the skin, hair and nails. It typically causes a raised, red, circular rash on the skin; symptoms appear within 4 to 14 days after being infected.
Symptoms of Dermatophytosis in Dogs
Dogs infected with dermatophytosis may show one or more of the following symptoms:
- Round raised areas of hair loss
- Irregular patches of hair loss
- Scaly or crusty skin
- Irritated and inflamed hair follicles
- Pustules on the skin
- Dry, brittle or cracked nails
- Secondary bacterial skin infections
Types
There are three most common fungal species that may cause ringworm in dogs; all three are zoonotic and may be transmitted from pet to human.
- Microsporum canis – Fungus that affects the upper layers of the skin on cats, dogs and humans
- Microsporum gypseum – Is a soil associated dermatophyte
- Trichophyton mentagrophytes – Affects 30% of dogs, especially Terrier breeds
Causes of Dermatophytosis in Dogs
- Dermatophytosis is contagious and is transmitted by direct exposure to the fungus
- It may be transferred by contact with an infected animal or human
- Dermatophytosis may also be passed by handling contaminated soil
- Fungal spores may transferred by a pet’s bedding, brushes and furniture
- Puppies, canines with a suppressed immune system, or pets on steroid medications are more susceptible to contracting dermatophytosis
- Contact with infected rodents may transmit dermatophytosis
- Flea bite may transmit the fungi
Diagnosis of Dermatophytosis in Dogs
Your veterinarian will take a thorough history of your pet. You will be asked questions on what the symptoms are and the timeline as to when they started. Your veterinarian will perform a physical exam. A common screening test used in dermatophytosis is the use of a specialized ultraviolet light called a Wood’s lamp. There are several types of the ringworm fungus that will glow with a yellow-green hue in a dark room when exposed to a Wood’s lamp. The Wood lamp is not always accurate because not all fungi fluoresce.
A more reliable test is to perform a fungal culture, using a sample of your pet’s skin or hair. There are special culture mediums designed specifically for identifying ringworm infections. A positive culture can sometimes be confirmed within a couple of days but some fungi take longer to grow. Your veterinarian may also examine your dog’s hair under a microscope to see if he can identify the fungal spores.
Treatments
Successful treatment of dermatophytosis in dogs and cats requires the combination of topical treatment, systemic treatment, and environmental disinfection.
Topical Treatments
Because dermatophytosis is transmitted by contact with arthrospores, topical therapy is an essential component of treatment. Topical treatment helps speed the resolution of infection and decrease the shedding of arthrospores into the environment.
Clipping
For decades, clipping has been a necessary part of dermatophytosis treatment; however, clipping is currently being reconsidered because whole-body clipping is stressful and the common microtrauma of the skin can worsen the infection. Thus, whether to clip should be decided on a case-by-case basis; clipping is not necessary for short-coated animals.
Dips, Shampoos, and Rinses
In the United States, lime sulfur dips are still recommended. Several studies have documented the efficacy of lime sulfur dips, and twice weekly application is more effective than once weekly.1,12-14 The dip has residual activity on the coat, whereas that of shampoos is shorter.13 In the author’s experience, common side effects of lime sulfur are dryness and yellow discoloration. Although older studies reported on the potential risk for oral ulcers in cats that lick their coat while still wet, newer studies have not confirmed these findings, raising the possibility that the older studies used more concentrated solutions.1 Most current lime sulfur formulations for veterinary use are 97.8% saturated lime sulfur, which is applied at a dilution of 8 oz/gallon of water.
When clients object to the smell of the sulfur dip, other options available in the United States include shampoos and rinses. Among the shampoos, neither chlorhexidine nor miconazole alone is considered an effective treatment. The most effective topical treatment is the combination of miconazole and chlorhexidine used twice weekly.15,16 Although clinicians commonly believe that chlorhexidine has antifungal properties, the efficacy of chlorhexidine for dermatophytosis has been shown to be poor.
Topical enilconazole is also effective for treating dermatophytosis in small animals18 but is currently not available in the United States. In countries where enilconazole rinse is available, it is considered a very effective topical option against dermatophytosis.19 Although enilconazole is generally well tolerated, it has been reported to cause hypersalivation, muscle weakness, and slightly elevated serum alanine aminotransferase (ALT) concentrations in Persian cats.
The efficacy of shampoos containing ketoconazole for animals has not been assessed by in vivo studies. In vitro studies support effectiveness of ketoconazole, but no clinical trial on ketoconazole shampoo for dermatophytosis in small animals has been published.
Similarly, no in vivo studies support the use of topical climbazole for dermatophytosis in small animals. In vitro studies showed good residual activity of 0.5% climbazole with chlorhexidine.
Only 1 study on topical terbinafine has been published, and it reported a good response.
Systemic Treatments
The ideal choices for systemic therapy are drugs that are keratinophilic and lipophilic and accumulate in the skin and keratin. Currently, the most effective systemic treatments for both dogs and cats are oral itraconazole or oral terbinafine.1
Itraconazole has a long half-life in cats21 and a great propensity to accumulate in hairs and skin. This property enables use of pulse therapy, which decreases the cost of therapy; daily administration for 1 week followed by 1 week on and 1 week off has shown clinical success.22 Others have started the pulse regimen only after giving the drug daily for 4 weeks.23 The most commonly used dose for itraconazole in dogs and cats is 5 mg/kg once daily. Because itraconazole affects cytochrome P450, it is important to consider drug interactions and decrease doses of other medications if their metabolism is affected by this interaction (e.g., cyclosporine).
Compounded formulations of itraconazole should be avoided; several studies showed that compounded formulations had subtherapeutic values in treated animals.24,25 Generic itraconazole seems to be better, although therapeutic monitoring is helpful because concentration can be very variable.If treatment failure is noted, switch to the product formulated for humans, Sporanox (Janssen Pharmaceuticals, janssen.com). Note also that when used at higher doses, itraconazole has been reported to trigger vasculitis in dogs. Clinicians should consider the possibility of this adverse effect when using this medication, although the high doses are typically reserved for systemic mycosis and are not recommended for dermatophytosis. A liquid formulation (Itrafungol; Elanco, elanco.com) is available for use in cats. Reported adverse effects of itraconazole include elevated liver enzymes and anorexia in dogs and decreased food consumption, depression, and elevated serum ALT concentrations in cats.
Terbinafine is very keratinophilic and accumulates in hairs, making it possible to do pulse therapy, which decreases cost and adverse effects. Terbinafine has excellent activity against dermatophytes, and one study has shown that it is efficacious and can represent a suitable and cheaper alternative for shelter cats. The commonly used dose range for terbinafine is 20 mg/kg to 40 mg/kg; efficacy is increased at higher doses. Although terbinafine does not have the same effect on cytochrome P450 as the azoles, its metabolism largely involves the liver; monitoring of liver values may be necessary when treatments are prolonged.
Ketoconazole is effective against dermatophytes, although it is not as good a treatment choice as itraconazole or terbinafine. Ketoconazole has been used in cats;29 however, because it is not typically well tolerated and frequently causes nausea and anorexia, it is best to reserve this medication for dogs. Ketoconazole is typically prescribed for dogs at 5 mg/kg PO q12h and is best administered with food to minimize adverse effects and increase absorption.
Fluconazole has poor activity against dermatophytes in vitro30 and is no longer recommended for treatment of dermatophytosis. Fluconazole is also water soluble and does not have the same ability as itraconazole and ketoconazole to accumulate in the skin and keratin.
Griseofulvin has historically been used to treat dermatophytosis but safer and more effective choices are now available. Thus, griseofulvin is rarely selected as a treatment.
Lufenuron was previously considered as a possible treatment, but studies have shown no efficacy. Therefore, lufenuron should not be considered as a treatment option.
Vaccines do not prevent development of dermatophytosis33 and thus should not be used for that purpose.
Environmental Decontamination
Environmental decontamination is a major part of dermatophytosis treatment. It also minimizes false-positive fungal culture results. Although separating animals for the purpose of minimizing contamination34 has been advocated for decades, confinement needs to be done with care because it can be very stressful, particularly for young animals.1 Thus, the duration of isolation should be minimized to what is needed to decontaminate the environment. The need for extended isolation can be decreased by weekly cleaning and use of topical therapy.
Studies have shown that weekly cleaning is very effective for removing infective arthrospores. The most important part of the decontamination process is the actual hard cleaning, which involves removal of debris and hairs. Cleaning can be accomplished with over-the-counter household detergents.37 Hard surfaces can be disinfected with 1:100 concentration household bleach or accelerated hydrogen peroxide. Machine washing of soft fabrics should be done by using the longest cycle, to maximize spore removal.
Monitoring Treatment Success
Clinical cure does not always equate to mycological cure. Thus, hair regrowth and the clinical appearance of the patient may not be sufficient criteria for decisions about duration of treatment. It is currently recommended that monitoring therapy and establishing whether a patient is completely cured should be based on a combination of resolution of clinical signs and a negative fungal culture. Extent of infection can be monitored by performing weekly cultures.
Dead fungal organisms can still be detected by PCR. Thus, a positive PCR result along with resolution of signs may indicate that the patient still has some spores on the coat rather than an actual infection. Patients may, for example, pick up spores from a contaminated environment although they have developed immunity and are no longer actively infected. These animals may represent a source of infection for other individuals around them, and further environmental decontamination may be warranted. Fungal cultures are easily available in practice and can help with treatment monitoring. When in doubt, fungal culture can be repeated to ensure that the results are truly negative.
Long-haired cats have a propensity for subclinical dermatophytosis39 and therefore are potential carriers. When dealing with an outbreak of dermatophytosis in a multicat household, every cat should be cultured to determine which are truly negative and which are clinically normal but carrying arthrospores on their coats and potentially acting as a source of infection for others.
Although dermatophytosis in dogs and cats will resolve without any therapeutic intervention, accurate diagnosis and the institution of appropriate treatment and environmental decontamination will shorten the duration of infection. This will benefi t the welfare of the affected animal itself and also reduce the risk of transmission to other animals and to its owners.
EDITED BY-DR. DN SINGH, PET CONSULTANT, BENGALURU
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